1. Field of the Invention
The present invention relates generally to antennas and, more particularly, to multibeam phased array antennas.
2. Background
The monostatic radar uses a transmit and receive beam of RF energy. The transmit beam illuminates a target scene. The received beam is formed from echoes of the transmitted energy that are reflected from the target. Energy in the sidelobe region of the transmit beam (energy not directed toward the target of interest) is reflected off additional obstacles and that energy is then received by the sidelobe portion of the receive beam causing a response that interferes with the signal from the desired target. The interference is called clutter. Clutter reduces the sensitivity of the radar system.
Multiple beam (multibeam) phased array antennas are well known and may be employed in a wide variety of applications. Applications that transmit to land or earth surface have sidelobes in the transmit and receive mode. The side lobes in the transmit mode radiate energy and illuminate targets that are off the main lobe of the beam axis (shown as 212 in FIG. 2) of radiation. Energy is returned from these off-axis targets along with energy from the on axis, main lobe. The energy returning from the off-axis targets is indistinguishable from energy returning from the main lobe. The energy from the off-axis beam appears in a display as clutter and noise. As the amount of clutter increases, it becomes more difficult, and finally impossible, to distinguish the targets that are of interest. The resolution of the subject radar is thereby impaired by the clutter contribution to the returned signal.
Signals received from sidelobe energy typically have 40 dB to 60 dB in gain below the gain of signals received by a main lobe. Taylor weighting is a method of assigning taper values to particular elements in an array to reduce the sidelobes of the transmitted beam.
In a typical SAR (Synthetic Aperture Array) radar, a particular target appears on the screen or in the data stream for processing for certain duration. During this time, the target may be captured several hundred times with successive transmissions. Data relating to the target is stored over the time interval. The data obtained relating to a target is summed or integrated giving the effect of having been painted by an antenna having a very large diameter. If the beam diameter is 10 feet and the integration process takes place over a mile of closing distance, the antenna is huge and the resolution which is the wavelength of the signal divided by the diameter becomes very small. For a SAR to operate in this manner, information received from sidelobe transmissions must be at least 60 dB below the product of the transmitted main lobe energy (TXmain) and received main lobe energy (RXmain). Signals received from sidelobe energy typically have 40 dB to 60 dB in gain below the gain of signals received by a main lobe.
Taylor weighting is a method of assigning taper values to particular elements in an array to reduce the sidelobes of the transmitted beam. A normal Taylor weighted taper is difficult to produce that provides sidelobe received signals that are 60 dB or more below comparable reflections from the TXmain and RXmain lobes.
Prior optimization methods were used to limit the sidelobe levels and clutter but these techniques optimized the transmit or receive beam separately in isolation. Specific features of the transmit beam, such as peaks and nulls were not exploited in the optimization of the receive beam characteristic. The problem of clutter isolation was limited by a failure to exploit a previous knowledge of the transmit beam, receive beam and scattering environment during optimization of the parameters of the receive and or transmit beam characteristics. Therefore, there is a need for a method and system for optimizing transmit/receive bema weights.